1. Field of the Invention
The present invention relates to a piezoelectric resonator and a method of manufacturing thereof, in which the piezoelectric resonator is adapted to be vibrated in a square type vibration mode for use in an oscillator, a filter, a discriminator, a ladder type filter, or other apparatuses.
2. Background of the Related Art
Referring to FIG. 14, a structure of a conventional piezoelectric resonator 51 is shown. The piezoelectric resonator 51 is adapted to be vibrated in a square type vibration mode. The piezoelectric resonator 51 includes a piezoelectric substrate 52 having a square shape and first and second flat main surfaces. A partial electrode 53 is disposed on the center of the first main surface of the piezoelectric substrate 52 and a whole-surface electrode 54 is disposed on the second main surface so as to cover an entire surface thereof.
The piezoelectric resonator 51 is characterized by its resonant frequency. Moreover, the piezoelectric resonator 51 has a damping property so that if the resonator 51 is used in a ladder filter, or other similar apparatus, the resonator 51 is characterized by its electrostatic capacitance. Therefore, when manufacturing the piezoelectric resonator 51, controlling the value of the resonant frequency or the electrostatic capacitance is essential. The electrostatic capacitance is controlled by adjusting the area of the partial electrode 53.
However, in the conventional piezoelectric resonator 51, it is necessary to provide electrically conductive paste on both main surfaces of the piezoelectric substrate 52 to define the electrodes. One of the electrodes is then patterned via etching to define the partial electrode 53. However, the outline of the partial electrode 53 becomes faded by bleeding and blurring of the resist ink that is used for the etching process. Further, the partial electrode 53 may become deformed by the distortion in the printed pattern of the resist ink. These manufacturing problems degrade the electrical properties of the piezoelectric resonator 51. Additionally, because the partial electrode 53 is defined by an etching process with the resist ink acting as the etching mask, the etching liquid corrodes the piezoelectric substrate 52. This may also degrade the electrical properties of the piezoelectric resonator 51.
One way to overcome the above-mentioned problems is by making a piezoelectric resonator 61 as shown in FIG. 15. The piezoelectric resonator 61 is made by dividing an electrode into partial electrode 63a for input-output, and defining other partial electrodes 63b (electrodes in which an electrical signal is not transmitted) by cutting grooves 64 in the piezoelectric substrate 62. This method does not require that etching be performed on the electrodes disposed on the main surfaces.
So by making a piezoelectric resonator as shown in FIG. 15, the above-mentioned problems caused by the etching process are overcome. Further, in the piezoelectric resonator 61, a space B, which is equal to the distance between the center lines of the grooves, is defined so that the desired electrostatic capacitance and a mechanical coupling coefficient is obtained. Moreover, a length L of one edge of the piezoelectric resonator 61 is defined so that the desired resonant frequency is obtained. Finally, to prevent the variation of the resonator characteristics which occurs in every piezoelectric resonator, the width and depth of the grooves are set at appropriate fixed values.
Note that when defining the grooves 64 to have a fixed depth in the piezoelectric resonator 61, the piezoelectric resonator 61 is first placed on a fixed board. The resonator 61 is then cut by horizontally moving a precise cutting apparatus, such as a cutting saw, to define the grooves 64. Referring to FIG. 16, the depth D of the groove 64 is equal to the difference between the total thickness T and the remaining thickness N of the piezoelectric resonator 61. Note that the depth D is equal to the distance from the surface of fixed board to the lowest point in the groove 64.
However, a significant drawback of the piezoelectric resonator 61 is that it is difficult and complicated to make. It is especially difficult to detect the position of the surface of the piezoelectric resonator 61 (the thickness of the piezoelectric substrate 62) every single time to define the groove 64 in the piezoelectric resonator 61, especially in a mass-production environment. Note that the variation in thickness of the piezoelectric substrate 62 can be comparatively large.
This drawback is illustrated in FIGS. 17A, 17B and 17C. If the thickness T1 of the piezoelectric resonator 61 is as small as shown in FIG. 17A, the depth D1 of the groove 64 of the piezoelectric resonator 61 becomes shallow, or if the thickness T2 of the piezoelectric resonator 61 is thick as shown in FIG. 17B, the depth D2 of the groove 64 of the piezoelectric resonator 61 becomes deep. Also, as shown in FIG. 17C, the thickness variation of the groove 64 that is cut into the piezoelectric substrate 61 is large when the piezoelectric substrate 62 has a curvature.
Therefore, in the above-described method, after the manufacture of a piezoelectric resonator, an additional step of resonance tuning is required. This results in increased difficulty, time and cost of manufacture and affects the resonance characteristics of the resonator.